1. Field of the Invention
The present invention relates generally to a mechanical burner assembly particularly intended for the continuous simultaneous combustion of two different hydrocarbon-based fuels which after being independently introduced to a flame, but fired together sustain combustion. In particular, this invention relates to a burner or other apparatus for continuously co-firing a suspension of fine-grained coal particles in a non-combustible liquid such as water with firing of pulverized coal.
2. Description of the Prior Art
In the coal burning industry coal particles are often referred to as fines and the size of the particles is defined by their capacity to pass through a particular mesh filter. Low cost coal fines or coal particles can be derived from particle coal streams and operating coal processing plants or coal ponds comprising a mixture of coal particles and water. The prior art has determined that the economics for the use of coal fines or particles favor the use of less than 100 mesh coal particles or coal fines which are classified or removed from a coarse coal stream of greater than 100 mesh fines or particles.
When less than 100 mesh coal fines or particles are processed into an aqueous slurry it is unnecessary to use cost-adding stabilizers, dispersoids or to utilize additional energy to produce the less than 100 mesh coal particles or ultrafine coal by grinding of larger pulverized coal pieces.
It is important for environmental reasons that wet fine coal slurry accumulation be carefully managed and consumed if possible. Accumulation can be prevented by initially separating fines from coarse coal during wet processing at mine sites, and transporting the fines in slurry pipelines to the point of utilization separately from the conventionally transported bulk coarse coal. Dust losses to the environment could be prevented by commercial economically viable burning of the slurry if such a burner and process were available. The capacity to economically burn a slurry will prevent the fines in the slurry from locating near the surface and drying and dusting off into the atmosphere. Burning of the slurry in the novel burner reduces the cost of energy from coal pulverization at the power plant. The energy cost is greatly reduced because of the use of the previously removed fines as fuel.
To eliminate environmentally undesirable dust losses, problem causing fines are initially removed from coal and later systematically re-mixed as a slurry. Some of the fines removal takes place at the mine site during wet processing.
It has been a persistent quest in the prior art to discover a way to burn a slurry recombined with the conventionally transported coarser pulverized coal. Heretofore, the effort was either to make the slurry capable of sustaining combustion alone or to mix the slurry with large pulverized pieces of coal before firing. The novel modified burner assembly avoids recombination and instead allows firing in the pulverized coal flame to take place at the point of combustion. The coal-water slurry is co-fired to fuel steam generators.
Heretofore, a burner assembly for the co-firing of coal-water slurries with pulverized coal on a commercial basis had not been achieved. From a utility company standpoint it is desirable to co-fire a coal water slurry fuel at, for example, the rate the slurry is produced from coal cleaning and by-product coal-water slurry production facility or to have a continuous supply from a reservoir. To be economically viable, it is necessary that a furnace for burning coal-water slurry be able to burn a slurry containing 50% solids for coal particles by weight or less.
In 1891, this technology started when Andrews was granted U.S. Pat. No. 449,102 which generally disclosed the mixing of coal dust with water to form a coal water slurry to be transported through pipes or the like to reservoirs, ponds or tanks. In 1936, Burke was issued U.S. Pat. No. 2,128,913 for an improvement in the coal water slurry process for transporting coal by changing its state to pumpable fluid by grinding the coal and mixing the disintegrated coal with water and a small amount of water soluble soap and then coagulating the suspending agent, for example, adding a mixture of lime to recover the coal.
U.S. Pat. No. 2,791,471 entitled "Transportation of Coal by Pipeline" described and claimed a coal water slurry comprising coal particles mixed with water having a spectrum of sizes with a nominal top size in the range of 6 to 28 mesh Tyler standard screen series and having less than 25% by weight of particles having a size greater than 14 mesh. The patent further indicated that the coal particles having the aforementioned size distribution and nominal top size were mixed with water to prepare a slurry comprising 35% to 55% by weight of the coal particles and the remainder water. The patent further disclosed that the slurry was pumped through a long distance pipe line at a linear velocity of between 4 to 7 feet per second and recovered at the delivery end of the pipeline.
U.S. Pat. No. 3,168,350 issued to Phinney in 1965 recognized that an important limitation of a slurry prepared according to the aforementioned recipe requires the expenditures of a significant amount of energy to convey the slurry through a long distance pipeline. Phinney also recognized that the slurry according to the recipe is unstable and that under static conditions the coal particles quickly settle out of the slurry as a highly immobile mass. Phinney was able to considerably reduce the energy necessary to convey the slurry described by blending in a prescribed manner two quantities of coal each having a different spectrum of particle sizes and mixing the blended coal particles with water. The synergistic discovery was that the energy requirements of the slurry prepared from the blended coal particles was less than the energy requirements to transfer slurries prepared from either of the two quantities having a different spectrum of sizes alone. In other words, what was discovered was that by blending two slurries each having a different spectrum of coal particles both with high transportation energy requirements a slurry was obtained which had a transportation energy requirement less than either of the two slurries originally employed in the blend. What Phinney discovered was to blend relatively coarse coal particles having a spectrum of sizes and a nominal top size of 4 mesh of the spectrum with relatively fine coal particles having a substantial quantity of particles smaller than 325 mesh.
Adams in U.S. Pat. No. 3,341,256 issued in 1967 and entitled "Process for Conveying Mineral Solids Through Conduits" describes the procedure at the time for pipeline transport of coal as first pulverizing the coals to a size range in which practically all the coal is less than 1/8 inch with approximately 30% passing through 200 mesh screen and then incorporating water to create a pumpable slurry. Adams pointed out that techniques had been devised to burn the slurry directly but the result was not commercially viable because the burning resulted in too great of a reduction of thermal efficiency. Adams developed a thixotropic mixture to defeat settling velocity of large coal particles from settliing rapidly through the low viscosity water of the slurry. This thixotropic means comprised a fluid or gel prepared from a mixture of water, leonardite, sodium hydroxide and calcium oxide. This thixotropic fluid in various proportions was discovered to be an excellent suspension medium for pipeline transportation of solid materials such as coal. Unfortunately, after transport, the solid materials, mainly the coal, had to be separated from the gel.
U.S. Pat. No. 3,589,314 issued in 1971 to Tretz for a method and device for pressure spraying and burning a coal dust-water mixture. Tretz pointed out that in 1971 experiments were being carried out internationally to develop methods for directly burning mixtures of coal dust and water in power plants. However, the then state of the art mixtures contained about 60% coal dust and 40% water by weight which causes continuous processing problems.
Tretz discloses the direct combustion of the then state of the art, heavily loaded, coal-water slurry after direct transport through a pipeline. The mixture, capable of sustaining combustion, was burned after spraying into a furnace or burner via a high pressure rotary sprayer or spray nozzle. The Tretz method burns a coal-water slurry where prior to discharging the slurry from a nozzle at a pressure of several atmospheres the slurry is heated to a temperature just below the water-vaporizing temperature upstream of the nozzle to pressure-relieve and vaporize the water in the slurry into steam directly after passing through the nozzle. The coal-water slurry is accumulated in a funnel shaped supply vessel from which it is supplied to the burners of a power plant upon demand. The supply vessel is adapted to stir or agitate its contents to prevent settling of the coal dust from the slurry. The coal-water slurry is preheated in a heat exchanger with superheated steam.
In another embodiment of the invention, the coal-water slurry is heated in a nozzle by a suitable electric heating device surrounding the nozzle. A temperature sensing device located down stream of the electric heater controls a regulated voltage source which provides energy to the heater adapted to the nozzle.
The Tretz method for burning a coal water slurry is limited to a highly loaded 60% to 40% prior art coal dust to water mixture by weight.
In August 1978 U.S. Pat. No. 4,094,035 issued to Cole et al contained a disclosure that a coal water slurry with more than 50 wt. % of coal was unpumpable. Liquid fuels, coal water slurries included, according to the prior art must be vaporized before they can be burned.
Large capacity industrial burners use two sequential steps, atomization plus vaporization, to get liquid carbonation fuel into a combustible form. Atomization is the process of breaking a liquid into a multitude of tiny droplets. By first atomizing the liquid carbonation fuel or coal-water slurry and thus exposing the large surface area of millions of tiny droplets to air and then to heat, atomizing burners are able to vaporize liquid carbon based fuel at very high rates. See, North American Combustion Handbook, Second Edition, North American Manufacturing Company, Cleveland, Ohio, 1978, pages 251 and 418.
Generally, the prior art discloses that the viscosity required for a coal-water slurry for effective atomization is substantially lower than the viscosity required to effectively pump the slurry. In summary, the prior art teaches that carbon based slurries containing more than about 50 wt. % coal cannot be effectively atomized and burned. Firstly, because they cannot be pumped to the atomizer and because of a solids contents of greater than 50 wt. % they are unpumpable. Secondly, even when the slurries have low enough viscosity to be pumpable they often have too high a viscosity to be effectively atomized and burned.
Funk discloses a process for burning a carbonaceous slurry having at least 55 volume % carbon material whereby the slurry is atomized and subsequently combusted. A burner which utilizes a coal-water slurry is described in a publication by T. M. Sommer and J. Funk entitled Development of a High-Solid Coal-Water Mixture for Application as a Boiler Fuel which was contributed by the Fuel Division of the American Society of Mechanical Engineers for presentation at the joint ASME/IEEE Power Generation Conference, Oct. 4-8, 1981, St. Louis, Mo. (pgs. 1-4); the disclosure of this publication is hereby incorporated by reference herein as prior art. The prior art coal-water slurry generally described by Funk is comprised of a fine or fine slurried product of a concentration of preferably more than about 50 wt. % of solids. A fine consist is combined with a coarse consist of coal particles having a mean particle size which exceeds 40 microns. The coarse and fine fractions are then combined with each other, a carrier liquid and a disbursing agent to produce a grinding mixture comprised of from about 60 to about 82 vol. percent of coal, from about 18 to about 40 volume percent of carrier liquid or water, and from about 0.01 to about 4.0 wt. % of a dispersing agent.
In 1981, Wiese was issued U.S. Pat. No. 4,304,572 for a method of producing a pumpable slurry with a high solids content. Wiese points out that coal as mined contains varying amounts of water which in some instance may range up to 40% by weight or even higher in the case of low grade coal. Wiese suggests that even this 40% water content is an undesirable constituent of the fuel. It is pointed out that if solid fuel is to be transported by pipeline in the form of a slurry, water trapped in the pores of the solid fuel which takes no part in the formation of the slurry are also transported. Thus, the slurry containing 50% by weight water and 50% by weight solid fuel would contain considerably less than that amount of fuel when the fuel therein is measured on a dry basis.
The amount of water necessary to form a pumpable slurry depends, according to Wiese, on the surface characteristics of the coal. In the case of a slurry which is to be fed to a gas generator, Wiese suggests it is necessary that the coal be ground to such an extent that a major portion thereof will pass through a 200 mesh sieve so that the particles are small enough to be substantially completely converted to oxides of carbon during their short residence time within the gasification zone in a gas fire furnace. Wiese teaches that for the slurry to be pumpable it must be made up of solid fuel particles most of which will pass through a 200 mesh sieve and that the coal water slurry contain from about 55 to 60 weight percent water. Wiese further points out that a slurry containing this amount of water renders the operation of the gas fired furnace unsatisfactory. The excess water moderates the temperature of the reaction zone to such an extent that it is thermally inefficient.
In 1984, Sawyer, Jr. was granted U.S. Pat. No. 4,432,771 for a combustible coal water slurry mixture and method for preparing same. The composition disclosed is a coal-water slurry containing 65 to 70 wt. % coal.
It was previously believed generally necessary to incorporate a maximum amount of coal in the slurry. The perceived solution to the problem of burning slurry in the prior art was to maximize the coal and maintain rheological characteristics that assure good stability and spray ability in the burner nozzle for combustion of the slurry by itself. The maximum solids in a coal-water slurry is suggested in the prior art to be about 70 to 75% and a satisfactory slurry is suggested to only be attained with relatively round particles, a high percentage of fine particles and a dispersing/wetting agent. For any medium to long range stability, a gelling agent that imparts gel properties to the continuous water phase according to the prior art, was required.
Forster in U.S. Pat. No. 4,444,126 issued in 1984 discloses an apparatus for combustion of a suspension of coal particles in water. Combustion air preheated to 550 degrees C. is forced into a coal-water slurry preheated to 100 degrees C. in a portion of a burner upstream of the burner flame. Air passes into the preheated suspension through a porous wall which is preheated and it separates the end portion of the air duct from the suspension duct. Preheating is done by recuperators, through which the combustion product gases from the furnace flow before being discharged at a temperature low enough for evaporating the condensed water.
Four months later, Scheffee in U.S. Pat. No. 4,465,495 disclosed a process wherein a high fuel value coal-water slurry is directly injected into a furnace as a combustible fuel for the express purpose of supplanting large quantities of increasingly expensive fuel oil used by utilities, factories, ships and other commercial enterprises. Scheffee teaches the slurry should be loaded with finely divided coal in amounts as high as 50 wt. % to 70 wt. % of the slurry. Scheffee is generally directed toward burning of highly loaded coal-water slurries which are fluidic dispersions characterized as thixotropic or Bingham fluids having a yield point.
In 1985, U.S. Pat. No. 4,501,205 was issued to Funk for a process for burning a coal- water slurry containing at least 60% by volume carbon solids. Funk suggests again that coal water slurries prepared with a carrier liquid are unstable at solids contents exceeding about 50 wt. %. Funk indicates the use of more than about 50 wt. % of coal in a coal-oil mixture has an adverse effect upon the pumpability of the mixture.
Keller, Jr. et al in U.S. Pat. No. 4,515,602 issued in 1985 describe another prior art composition containing coal and water which can be used as a fuel. Keller correctly points out that it is part of the prior art that dwindling supplies of petroleum and natural gas and concerns about the regular availability of those products from foreign sources have led to increased interest by utilities and other consumers in the use of coal water slurries as an alternative fuel. Coal-water slurries reported in the patent and open literature for the most part have a particle size distribution of 60-80 plus or minus 200 mesh (74 microns).times.0 and ash contents of 3 plus to over 10 wt. %. Such mixtures are described in the patents previously discussed herein and in the following papers, all presented at the Fourth International Symposium on Coal Slurry Composition, Orlando, Fla., May 10-12, 1982: K. Aoki et. al, Pre-treatment of Coal for Coal Water Slurries, Sumitomo Heavy Industries, Ltd.; R. Ebri et. al, Coal-Oil Mixture and Coal-Water Mixture Fuels for Steam Generators, Combustion Engineering, Inc. G. Germane et. al, Coal-Water Mixture Combustion Studies in a Laboratory Cylindrical Combustor, Combustion Laboratory, Brigham Young University , Ghassemzadeh et. al, Rheology and Combustion Characteristics of Coal-Water Mixtures, Babcock and Wilcox Company; and R. Scheffee et. al for The Development of an Evaluation of Coal-water Mixture Technology, Atlantic Research Corporation. The Keller coal-water slurry disclosed has a particle size distribution of less than or equal to 30 microns.times.0 and an ash content that can range down to 1.5 to less than 0.3 wt. %. In one example, Keller points out that the resulting coal water slurry has a solid concentration of 50 wt. % and an absolute viscosity of 3300 centipoises at a shear rate of 5.sup.-1 seconds.
Tratz used a nozzle to spray a preheated slurry at several atmospheres into a combustion chamber and was predisposed to the notion that the nozzle and line would clog up unless the slurry was preheated.
Forster developed and patented a burner directly solely to the combustion of a preheated aqueous coal slurry.
As late as 1989, Siwersson was still approaching the problem of combusting a coal-water slurry as one wherein the slurry had to sustain its own combustion in the same manner as oil. Siwersson of Sweden patented a burner for an aqueous fine-grain coal solution which directed the slurry to a baffle opening to a rotating cup so that the aqueous slurry flowed outward from the baffle by centrifugal force.
A prior art rotary burner, as well as other known rotary burners, for oil has proved to be practically unusable, since, on the one hand, the fine-grained suspension showed a tendency to plug the flow channels and, on the other, the suspended particles had a tendency to stick to the inner side of the rotating burner cup and be burnt thereto because of too high of a solids content and the persistent effort to make the slurry capable of self sustaining combustion. A known oil burner type operates according to the so-called toroidal principle where the oil mist sprayed out from the nozzle is surrounded by a conically diverging air stream which, by a kind of ejector effect, produces a recirculation of the combustion gases inwards towards the oil burner nozzle. Attempts to use this known oil burner type for the combustion of the above mentioned special fuel in the form of a suspension of fine-grained coal particles in a liquid have also failed mainly because of the perceived necessity for self-sustaining combustion.
German Patent Specification No. 594,722 discloses a vertical oil burner in which the fuel is supplied by self-priming to the mouth of a pipe which extends into a rotary cup and terminates above the bottom thereof, such that the fuel is expelled towards the edge of the cup so as to be distributed by this edge into an air stream ascending around the rotary cup. Oil drops that are not entrained by the air stream are caught by a conical screen and flow down into an oil collector against the action of the ascending air stream which is produced by means of an annular nozzle disposed beneath the rotary cup. This prior-art oil burner rather operates in accordance with the rotary burner principle but not according to the above-mentioned toroidal principle since the gas velocity at the edge of the rotary cup is so low that it permits oil drops both to hit the surrounding screen and to descend along this screen. This known burner is not usable for a slurry in the form of a suspension of fine-grained coal particles in a liquid having less than 70 weight % solids either.
According to Siwersson et. al, by combining the per se known rotary burner principle with the per se known toroidal burner principle, it is possible to provide a burner which readily permits combustion of a slurry provided the slurry is structured for self-sustaining combustion, i.e., having about 70 weight % coal fines content by weight and about 30 weight % water.
Over the years, different kinds of such fuels have been proposed, but for these fuels to be economically advantageous, until the present invention, it was essential that the amount of liquid in the suspension be kept low. However, the lower the liquid content is, the greater are the difficulties of handling the slurry as a fuel. A type of such a coal suspension developed to solve difficulties associated with high solids content slurries is described in U.S. patent application. Ser. No. 908,497. The fuel disclosed therein consists of a very finely divided coal dust suspended in a liquid which is usually water but which may also be combustible in itself. This liquid fuel contains a suspending agent for maintaining the coal powder particles in suspension. This fuel consists of about 70% by weight of coal, about 30% by weight of water, and a small amount of suspending or dispersing agent, for instance 0.3% by weight, calculated on the whole of the fuel. The viscosity of the fuel may amount to 2500 cP Brookfield, and the particle size of the coal typically is about 50 .mu.m. The thermal value of the fuel typically is 21-25 MJ/kg (5.8-6.9 kWh/kg). A certain amount of fine-grained lime may also have been added to the fuel in order to neutralize the sulphur content of the coal.
This fluid suspended fuel was developed as a substitute for oil and gas but it gives rise to difficulties when burnt because of the tendency of the fuel to choke channels and the like. Attempts to use this combustible suspension in conventional oil and gas burners, have been met with serious problems. Plugging of nozzle orifices has been a primary problem.
Thus, heretofore, engineers made the slurry independently combustible or capable of self-sustaining combustion. Subsequently, unsuccessful attempts were made to use the combustible suspension in conventional oil and gas burners.
As evinced by the foregoing prior art references, engineers and scientists have been attempting to solve the problem of how to consume aqueous solutions of fine-grained coal by defining those physical attributes of a composition which allow easy transport and which sustain independent combustion rather than creating a burner which accommodates a slurry as it is commonly available or normally and customarily produced.
Hence, according to the present invention, it has quite surprisingly been discovered that by combining direct input of a slurry containing a high water content, as high 62 weight % water, using a conical spray nozzle for delivery via a direct pressurized tube extending through a burner customarily adapted to fire pulverized coal, as long as the pulverized coal is initially fired, the subsequently introduced slurry co-fires in the burner with the pulverized coal without any difficulty whatsoever.
The novel burner assembly has been demonstrated in a commercial boiler using six burners, three of which were structurally modified according to the principles and concepts of the invention as set forth herein.
Remarkably, a long-felt need in the industry has been met by a relatively simply solution. A novel burner arrangement has been discovered, tested, and implemented which enables economically profitable, commercial burning of slurries containing from 38 to 70 weight percent coal fines and reciprocally from 62 to 30 weight percent water, respectively, by co-firing the slurry with pulverized coal.